During the production of base metals, particularly aluminum metal, various oxides, nitrides, and carbides form during the melting processing and create a byproduct known as slag or dross. When dross forms, it contains byproducts other than the desired metallic end product or base metal, but may also contain between 30% -70% of the base metal, depending on the particular process and type of furnace. While it is primarily desirable to remove the dross and separate it from the molten base metal, it is also desirable to then recover as much base metal as possible from the removed dross. Indeed, plants which remelt the base metal have found that recovering metal from the dross can significantly increase the overall output of the production process.
In the aluminum production process, dross floats on top of the molten aluminum in the production furnace. When skimmed from the production furnace, the temperature of the dross typically ranges between 1,300.degree. to 1,800.degree. fahrenheit (F). Dross at such elevated temperatures tends to react with moisture, resulting in various emissions and odor considerations. Further, the aluminum base metal in the hot dross often reacts immediately with the surrounding atmosphere and burns or thermites. The temperature of the thermiting dross can reach approximately 3,000.degree. F. and can consume approximately 1% of the aluminum by-weight per minute.
Traditional techniques directed to cooling the aluminum dross often require expensive equipment and are marginally effective at accommodating the hot dross at the above-discussed elevated temperatures. Further, because the hot dross cannot practically be transported from the metal production site to a remote dross processing site, the dross typically must be cooled at the metal production site.
While this process enables limited recovery of the base metal, the desire to increase residual aluminum recovered from the dross has required procedures which provide more effective recovery of the base metal than provided by processes which merely allow the dross to burn itself out. Consequently, salt may be added to the dross to expedite the cooling process and reduce thermiting in an effort to improve the metal recovery process significantly. In a variation of the dross cooling process, dross is spread in a shed so that the emitted exhaust may be collected and processed in a contained environment, thereby eliminating any emissions into the atmosphere.
Another approach to processing hot dross involves first cooling then harvesting the cooled, hardened dross using the methods described above. The harvested dross is then reheated in a rotary or plasma furnace in order to separate the base metal (typically aluminum) from the dross. One major disadvantage of this approach is that significant energy is expended by reheating the cooled dross. A variation of this dross handling process limits the energy loss resulting from cooling the hot dross by processing the hot dross at the first available opportunity. This significantly reduces the time available for thermiting, decreases the time for fugitive emissions thereby significantly decreasing the quantity of such fugitive emissions, and reduces the energy costs associated with reheating the cold dross.
There are several disadvantages, however, in processing hot dross immediately after it is skimmed. First, rotary furnaces typically used to process the hot dross operate more efficiently when processing large batches of dross, rather than the quantity of dross yielded by a single typical skimming operation. Second, irregular skimming periods limit effective batch processing because the skimmed dross currently cannot be stored for a sufficient time to accumulate a suitable quantity of material for efficient batch processing. Third, the dross processing furnaces must be in relatively close proximity to the dross producing sources. More particularly, because most existing facilities make use of a rotary type furnace, dross processing is further complicated. These furnaces are particularly susceptible to the inherent disadvantages described above because they require a large batch of dross for efficient operation.
Thus, it would be highly desirable to provide a dross processing apparatus and method in which hot dross can be stored at an elevated temperature in order to accumulate a sufficiently large batch of dross to effect efficient processing by a dross processing furnace. It would also be highly desirable to provide a method and apparatus for storing hot dross which does not permit appreciable cooling of the dross prior to charging it into a processing furnace, and which further substantially reduces or eliminates thermiting while the hot dross is being held prior to being charged into a furnace.